.container {
    /* 去除元素间的空格 */
    font-size: 0;
}

.left {
    position: relative;
    display: inline-block;
    width: 490px;
    height: 510px;
    margin-right: 16px;
    border: 1px solid #eee;

    background-repeat: no-repeat;
    background-position: center;
    background-size: cover;
}

.right {
    display: inline-block;
    width: 490px;
    height: 510px;
    border: 1px solid #eee;
    opacity: 0;
    background-repeat: no-repeat;

}

.dots {
    width: 490px;
    margin-top: 10px;
    text-align: center;
}

ul {
    display: inline-block;
}

.dots li {
    display: inline-block;
    width: 60px;
    height: 60px;
    margin: 0px 5px;
    background-repeat: no-repeat;
    background-position: center;
    background-size: cover;
}

.dots li:nth-child(1) {
    background-image: url("images/imgA_1.jpg");
}

.dots li:nth-child(2) {
    background-image: url("images/imgB_1.jpg");
}

.dots li:nth-child(3) {
    background-image: url("images/imgC_1.jpg");
}

.dots .check {
    border: 2px solid black;
}

.mask {
    position: absolute;
    top: 0;
    left: 0;
    width: 230px;
    height: 230px;
    background-image: url("images/bg.png");
    opacity: 0;
}